Electro-Conductive Paste for Forming an Electrode of a Solar Cell Device, a Solar Cell Device and Method for Producing the Solar Cell Device

ABSTRACT

In order to provide an electro-conductive paste bringing no increase of the contact resistance for forming an electrode of a solar cell device, the electro-conductive paste is characterized by containing an electro-conductive particle, an organic binder, a solvent, a glass frit, and an organic compound including alkaline earth metal, a metal with a low melting point or a compound affiliated with a metal with a low melting point.

TECHNICAL FIELD

The present invention relates to an electro-conductive paste for formingan electrode of a solar cell device, the solar cell device having theelectrode formed by the electro-conductive paste and a method forproducing the solar cell device.

BACKGROUND ART

As shown in FIG. 4, a conventional solar cell device is provided with asemiconductor board 21, a diffusion layer 22, an antireflection layer23, a back electrode 24, and a surface electrode 25.

For example, the solar cell device is produced as described below. Thediffusion layer 22 comprising impurity, and the insulatingantireflection layer 23 consisting of silicon nitride, silicon oxide,titanium oxide and the like are layered upon a light-absorbing side(surface side) of the semiconductor board 21 consisting of silicon inorder. The semiconductor board 21 contains semiconductor impurity suchas boron of 1×10¹⁶-10¹⁸ atoms/cm³ and presents one electro-conductivetype (e.g., p-type) with specific resistance of about 1.5 Ωcm. Singlecrystal silicon is produced by a pulling up method and poly crystalsilicon is produced by a casting method. Poly crystal silicon can bemass-produced and poly crystal silicon is more advantageous than singlecrystal silicon in production cost. The semiconductor board 21 can beobtained by slicing a piece with a thickness of about 100 to 300 μm ofingot produced by the pulling up method or the casting method.

The diffusion layer 22 is formed by diffusing impurity such asphosphorus on the light-absorbing side of the semiconductor board 21 andpresents contrary electro-conductive type (e.g., n-type) to thesemiconductor board 21. The diffusion layer 22 is, for example, formedby placing the semiconductor board 21 in the inside of a furnace andheating the semiconductor board 21 in a gas containing phosphorusoxychloride (POCl₃).

The antireflection layer 23 is formed on the light-absorbing side of thediffusion layer 22 in order to possess the antireflection function andprotect the solar cell device. The antireflection layer 23, which isconsisted of silicon nitride membrane, for example, is formed by plasmaenhanced chemical vapor deposition in which a mixing gas of silane(SiH₄) and ammonia (NH₃) becomes plasma by the glow-discharge and theresultant matter is accumulated. For example, taking consideration of adifference of refractive index from the semiconductor board 21, therefractive index of the antireflection layer 23 is made to a range ofabout 1.8 to 2.3 and the thickness of the antireflection layer 23 ismade to a range of about 0.05 μm to 1.0 μm.

The surface electrode 25 is formed on the surface of the semiconductorboard 21 and the back electrode 24 is formed on the back of thesemiconductor board 21. The surface electrode 25 is formed by printing,drying and sintering an electro-conductive paste which contains anelectro-conductive particle, an organic binder, a solvent, a glass fritand an optional matter. The back electrode 24 is also formed byprinting, drying and sintering the electro-conductive paste. It is notnecessary for the back electrode 24 to use the same electro-conductivepaste as the surface electrode 25. Particularly, the surface electrode25 plays a part as fire-through and it is important for improving thefunctions of the solar cell to adopt an appropriate composition and anappropriate sintering condition of the surface electrode 25. Thisfire-through is a following phenomenon:

In sintering, the glass frit contained in the electro-conductive pasteworks for the antireflection layer 23 and the antireflection layer 23 isdissolved. As a result, the surface electrode 25 and the diffusion layer22 touch each other and an ohmic contact is obtained between the surfaceelectrode 25 and the diffusion layer 22.

If the stable ohmic contact is not obtained between the surfaceelectrode 25 and the diffusion layer 22, an electric resistance inseries increases and a curved factor (FF) tends to decrease. Since thetransformation efficiency of the solar cell is obtained by multiplyingopen-voltage by short-circuit electric current density and FF, if FFbecomes smaller, the transformation efficiency of the solar cell isdecreased.

Well, the properties of the cell are important for enhancing the powercharacteristic of the solar cell. For example, the power efficiency isincreased by decreasing the electric resistance of the electrode. Inorder to attain the object, the following electro-conductive paste isdisclosed in patent publication 1:

The electro-conductive paste contains an organic binder, a solvent, aglass frit, an electro-conductive particle, at least one metal or ametallic compound selected from a group of Ti, Bi, Zn, Y, In and Mo. Theaverage particle diameter of the metal or the metallic compound is above0.001 μm and under 0.1 μm.

Patent publication 1 states that it is possible to form a surfaceelectrode having a stable high electro-conductivity and a good adhesionbetween the semiconductor and the electro-conductive paste byintervention of the antireflection layer, as a result of sintering ofthe electro-conductive paste containing the super-minute metal particleor the metallic compound. But, depending on the composition of theelectro-conductive paste, as particularly shown in patent publication 1,after printing and drying the electro-conductive paste containing thesuper-minute particle metal or the metallic compound on the surface ofthe semiconductor board, the coating film (the paste film) shrinks andthe contact resistance is increased by sintering. In one case, microcrack may be generated the surface of the semiconductor board bydifference of thermal shrinkage action (coefficient of linear expansion)between the paste film and the semiconductor board. If the contactresistance is increased, as described above, FF becomes smaller and thetransformation efficiency of the solar cell is decreased.

PRIOR ART PUBLICATION Patent Publication

Patent Publication 1: Japanese Laid-open Patent Application PublicationNo. 2005-243500

SUMMARY OF INVENTION Problems to be Solved by the Invention

In view of the forgoing, the object of the present invention is toprovide an electro-conductive paste bringing no increase of the contactresistance for forming an electrode of a solar cell device, a solar celldevice having the electrode formed by the electro-conductive paste and amethod for producing the solar cell device.

Means for Solving the Problems

In order to achieve the object, an electro-conductive paste for formingan electrode of a solar cell device of the present invention ischaracterized by containing an electro-conductive particle, an organicbinder, a solvent, a glass frit, and an organic compound includingalkaline earth metal

The electro-conductive paste for forming an electrode of a solar celldevice of the present invention is also characterized by containing anelectro-conductive particle, an organic binder, a solvent, a glass frit,and a metal with a low melting point

Further, the electro-conductive paste for forming an electrode of asolar cell device of the present invention is also characterized bycontaining an electro-conductive particle, an organic binder, a solvent,a glass frit, and an organic compound including alkaline earth metal anda metal with a low melting point.

Still further, the electro-conductive paste for forming an electrode ofa solar cell device of the present invention is also characterized bycontaining an electro-conductive particle, an organic binder, a solvent,a glass frit, and a compound affiliated with a metal with a low meltingpoint

Further, the electro-conductive paste for forming an electrode of asolar cell device of the present invention is also characterized bycontaining an electro-conductive particle, an organic binder, a solvent,a glass frit, an organic compound including alkaline earth metal and acompound affiliated with a metal with a low melting point.

Preferably, the organic compound including alkaline earth metal may bean alkaline earth metal soap.

Preferably, the alkaline earth metal soap may be magnesium stearate.

Preferably, the metal with a low melting point may be Te or Se.

Preferably, the compound affiliated with a metal with a low meltingpoint may be TeO₂.

Preferably, the electro-conductive paste for forming an electrode of asolar cell device of the present invention contains an organic compoundincluding an alkaline earth metal of 0.1 to 5 percent by weight.

Preferably, the electro-conductive paste for forming an electrode of asolar cell device of the present invention contains a metal with a lowmelting point of 0.1 to 5 percent by weight.

Preferably, the electro-conductive paste for forming an electrode of asolar cell device of the present invention contains an organic compoundincluding an alkaline earth metal of 0.1 to 5 percent by weight and ametal with a low melting point of 0.1 to 5 percent by weight.

Preferably, the electro-conductive paste for forming an electrode of asolar cell device of the present invention contains a compoundaffiliated with a metal with a low melting point of 0.01 to 10 percentby weight.

Preferably, the electro-conductive paste for forming an electrode of asolar cell device of the present invention contains an organic compoundincluding an alkaline earth metal of 0.1 to 5 percent by weight and acompound affiliated with a metal with a low melting point of 0.01 to 10percent by weight.

A solar cell device of the present invention is characterized in that adiffusion layer is formed on a light-absorbing side of a semiconductorboard, an antireflection layer and a surface electrode are formed on thediffusion layer and a back electrode is formed on the other side of thelight-absorbing side of the semiconductor board, wherein the surfaceelectrode is formed by printing and sintering the aboveelectro-conductive paste on the antireflection layer.

A method for producing the solar cell device of the present invention ischaracterized in that a diffusion layer is formed on a light-absorbingside of a semiconductor board, an antireflection layer is formed on thediffusion layer, the above electro-conductive paste is printed on theantireflection layer, an electro-conductive paste for forming a backelectrode is printed on the other side of the light-absorbing side ofthe semiconductor board, the electro-conductive paste printed on theantireflection layer is sintered to form a surface electrode by beingconnected to the diffusion layer and the electro-conductive paste forforming a back electrode is sintered to form a back electrode.

Effects of the Invention

In accordance with the present invention, it is possible to provide anelectro-conductive paste bringing no increase of the contact resistancefor forming an electrode of a solar cell device, a solar cell devicehaving the electrode formed by the electro-conductive paste and a methodfor producing the solar cell device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a section view showing an embodiment of a solar cell device ofthe present invention.

FIG. 2( a) is a plan view showing a light-absorbing side of anembodiment of a solar cell device of the present invention.

FIG. 2( b) is a plan view showing the other side of the light-absorbingside of an embodiment of a solar cell device of the present invention.

FIG. 3 is a section view showing an example of a solar cell module.

FIG. 4 is a section view showing a schematic constitution of aconventional solar cell device.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(1) Electro-Conductive Particle

As examples of electro-conductive particles, silver powders, silveroxide powders, silver carbonate powders, silver acetate powders, powderscoated with silver, silver-containing alloy powders, nickel powders,copper powders and the like are given. Single or a mixture of two ormore thereof can be used.

Preferably, the electro-conductive particles contain silver of 70 to 90percent by mass. In case that the electro-conductive paste printed onthe antireflection layer is sintered to be connected to the diffusionlayer, the sintering temperature is a temperature in a range of about750 to 900° C. Even if non-reducing atmosphere is applied to silver, theelectro-conductivity may be not decreased by surface-oxidation.

Preferably, a compounding quantity of the electro-conductive particlesmay be 65 to 90 percent by weight to the whole electro-conductive paste.Under 65 percent by weight, the compounding quantity of theelectro-conductive particles is too little and a resistivity of thelight-absorbing side electrode obtained by sintering is increased. Above95 percent by weight, a printing function is lowered and a physicaladhesion strength becomes insufficient.

The shape of the electro-conductive particles may be scale-shaped,spherical-shaped, flake-shaped, indefinite-shaped and a mixture thereof.

Preferably, an average particle diameter of the electro-conductiveparticle may be a range of 0.1 to 15 μm because it affects the sinteringcharacteristic (a larger electro-conductive particle is sintered slowerthan a smaller electro-conductive particle). Under 0.1 μm, a sinteringspeed is too fast and physical adhesion strength becomes insufficient.Above 15 μm, a sintering speed may be a little slow but a dispersionfunction and a printing function in the paste become bad. As a result,it is difficult to print a fine line. In this description, an averageparticle diameter denotes a particle diameter of a cumulative 50 percentaccumulated from small diameter side, in case that the particle diameterhas been measured by the micro-track particle size distributionmeasuring method.

Preferably, a specific surface area of the electro-conductive particlemay be a range of 0.05 to 5 m²/g. Under 0.05 m²/g, the particle diameterbecomes large and it is not possible to print a fine line. Above 5 m²/g,a large amount of solvent is needed to adjust the viscosity and the workefficiency is lowered.

(2) Glass Frit

Preferably, a glass frit which can be used in the present invention hasa softening point of temperature in a range of 300 to 550° C. so that anantireflection layer may be eroded and an appropriate connection to asemiconductor board may be conducted when the electro-conductiveparticle is sintered at a temperature in a range of 750 to 950° C. Ifthe softening point is lower than 300° C., the sintering is excessivelyprogressing and it is not possible to get the advantages of the presentinvention. On the other hand, if the softening point is higher than 550°C., a sufficient melting fluidizing does not occur at a time ofsintering and it is not possible to get sufficient adhesion strength.For example, as examples of glass frits, Bi-containing glass,Bi₂O₃—B₂O₃—ZnO-containing glass, Bi₂O₃—B₂O₃ containing glass,Bi₂O₃—B₂O₃—SiO₂ containing glass, Ba containing glass and BaO—B₂O₃—ZnOcontaining glass and the like can be used.

The shape of the glass frit may be spherical-shaped orindefinite-shaped.

Preferably, a compounding quantity of the glass frits may be a range of0.1 to 10 percent by weight to the whole electro-conductive paste. Under0.1 percent by weight, an adhesion strength becomes insufficient. Above10 percent by weight, a flotation of glass and a poor soldering occur.

As the detailed properties of glass frits, Bi-containing glass frit maycontain B₂O₃ of 1 to 10 percent by weight, BaO of 1 to 10 percent byweight, Bi₂O₃ of 70 to 80 percent by weight, Sb₂O₃ of 1 percent byweight or less, and the others (except said matters, ZnO and CaO) of 10percent by weight or less and its softening point may be a temperatureof about 430° C. Ba-containing glass frit may contain B₂O₃ of 20 percentby weight or more, ZnO of 20 percent by weight or less, BaO of 40percent by weight or more, and CaO of 10 percent by weight or more andits softening point may be a temperature of about 530° C.

(3) An Organic Compound including Alkaline Earth Metal, A Metal with aLow Melting Point and A Compound Affiliated with a Metal with a LowMelting Point

An organic compound including alkaline earth metal and a metal with alow melting point function as sintering controlling agent. A metal witha low melting point function is a metal having melting temperature of500° C. or less. Examples of the metal with a low melting point are zinc(melting point of 419.6° C.), lead (melting point of 327.5° C.), tin(melting point of 231.97° C.), bismuth (melting point of 271.3° C.),tellurium (melting point of 449.5° C.) and selenium (melting point of217° C.). Among those metals, preferably tellurium of semiconductorwhose specific resistance is 0.436 Ωcm may be used. Furthermore, inplace of metal with a low melting point, a matter having melting pointlower than sintering temperature of the electro-conductive paste, e.g.tellurium dioxide (melting point of 732.6° C.) may be used. That is, inplace of metal with a low melting point, can be used a matter havingmelting point lower than sintering temperature of the electro-conductivepaste (a compound affiliated with a metal with a low melting point suchas tellurium dioxide) or a matter showing chemical reaction (physicaltransformation) at a temperature lower than sintering temperature of theelectro-conductive paste.

As an organic compound including alkaline earth metal, preferablyalkaline earth metal soap may be used. Calcium stearate, magnesiumstearate, strontium stearate and magnesium gluconate and the like may beused as concrete alkaline earth metal soap.

The alkaline earth metal soap has a high surface activity and dispersesuniformly the electro-conductive particles as a kind of protectivecolloid and controls an excessive sintering of the elector-conductiveparticles at sintering in the air atmosphere. As a result, it can beexpected that the elector-conductive particles shows no much differentthermal shrinkage action from silicon of semimetallic element which ismain component of the semiconductor board. Further, if theelectro-conductive paste contains a metal with a low melting point or acompound affiliated with a metal with a low melting point, the metalwith a low melting point or the compound affiliated with a metal with alow melting point is liable to be oxidized when the electro-conductivepaste is sintered in the air atmosphere. That is, an oxidation film isnaturally formed at the sintering process of the electro-conductivepaste and an excessive sintering of the electro-conductive particles iscontrolled and it can be expected that the electro-conductive particlesshows no much different thermal shrinkage action from silicon ofsemimetallic element which is main component of the semiconductor board.Thus, after the electro-conductive paste of the present invention isprinted and dried on the surface of the semiconductor board, amicro-crack is not generated or a contact resistance is not increased atthe time of sintering.

In order to obtain the above advantage, preferably theelectro-conductive paste may contain an organic compound includingalkaline earth metal of adequate quantity, a metal with a low meltingpoint of adequate quantity, an organic compound including alkaline earthmetal of adequate quantity and a metal with a low melting point ofadequate quantity, a compound affiliated with a metal with a low meltingpoint of adequate quantity, and an organic compound including alkalineearth metal of adequate quantity and a compound affiliated with a metalwith a low melting point of adequate quantity. Specifically, preferablythe electro-conductive paste may contain an organic compound includingalkaline earth metal of 0.1 to 5 percent by weight. Further, preferablythe electro-conductive paste may contain a metal with a low meltingpoint of 0.1 to 5 percent by weight. Still further, preferably theelectro-conductive paste may contain an organic compound includingalkaline earth metal of 0.1 to 5 percent by weight and a metal with alow melting point of 0.1 to 5 percent by weight. Further, preferably theelectro-conductive paste may contain a compound affiliated with a metalwith a low melting point of 0.10 to 10 percent by weight, morepreferably 0.1 to 8 percent by weight, still more preferably 0.1 to 4percent by weight. Still further, preferably the electro-conductivepaste may contain an organic compound including alkaline earth metal of0.1 to 5 percent by weight and a compound affiliated with a metal with alow melting point of 0.10 to 10 percent by weight, more preferably 0.1to 8 percent by weight, still more preferably 0.1 to 4 percent byweight. Under the lower limit of the above numerical range, an excessivesintering of the electro-conductive particles cannot be controlled.Above the upper limit of the above numerical range, an electricresistance is increased and FF value becomes lower.

Compounds of selenium such as ferroselenium, alloy selenide, seleniumdioxide, selenious acid salt, selenic acid salt, selenium disulfide,selenium organic metallic compound and the like may be contained in theelectro-conductive paste.

Further, compounds of tellurium such as the following inorganiccompounds of tellurium or organic compounds of tellurium may becontained in the electro-conductive paste:

Tellurium chloride, tellurium dioxide, tellurious acid compound, zinctelluride, tellurium tetrabromide, aluminum telluride, cadmiumtelluride, hydrogen telluride, potassium telluride, sodium telluride,gallium telluride, silver telluride, chromium telluride, germaniumtelluride, cobalt telluride, mercury telluride, tin telluride, tungstentelluride, titanium telluride, copper telluride, lead telluride, bismuthtelluride, arsenic telluride, manganese telluride, molybdenum telluride,telluric acid, ammonium metatelluric acid, potassium metatelluric acid,rubidium metatelluric acid, sodium metatelluric acid, lead metatelluricacid, tellurium iodide, tellurium sulfide, diphenylditellurido,tellurium octylate.

Either compound of selenium and tellurium selected from the abovecompounds or both compounds of selenium and tellurium may be containedin the electro-conductive paste.

Particularly, ZnTe having the minimum energy gap (energy gap between thevalence band and the empty band) of 2.26 eV is a direct bandgap compoundsemiconductor and its impurity level obtained by addition of silver is0.11 eV away from the top of the valence band and relatively near thetop of the valence band. If an electron has no energy enough to gobeyond the forbidden band between the valence band and the empty bandand move to empty band, the electron stays at the valence band and theelectrical conduction does not occur. The electrical conduction occursby the electron's jumping over the valence band or the impurity level byabsorption of heat or light. In this point, small energy gap between thevalence band or the impurity level is advantageous and particularly,ZnTe may be preferably used among compounds of tellurium.

(4) Organic Binder

As organic binders, the following matters may be used but not limited:

Cellulose derivatives such as methylcellulose, ethylcellulose and thelike, acrylic resin, alkyd resin, polypropylene resin, polyvinylchloride plastics, polyurethane resin, rosin resin, polyterpene,phenolic resin, aliphatic petroleum resin, acrylic esters, xyleneformaldehyde resin, cumarone-indene copolymer, styrene resin,dicyclopentadiene, polybutene, poly ether, urea formaldehyde resin,melamine formaldehyde resin, poly vinyl acetate plastics, poly isobutyl.

Preferably, a compounding quantity of the organic binders may be a rangeof 0.1 to 30 percent by weight to the whole electro-conductive paste.Under 0.1 percent by weight, sufficient adhesion strength cannot beobtained. Above 30 percent by weight, a printing function is lowered dueto increase of paste viscosity.

(5) Solvents

As solvents, the following matters may be used but not limited:

Hexane, toluene, ethyl cellosolve, cyclohexanone, butyl cellosolve,butyl cellosolve acetate, butyl carbitol, butyl carbitol acetate,diethylene glycol diethyl ether, diacetone alcohol, terpineol, methylethyl ketone, benzyl alcohol.

Preferably, a compounding quantity of the solvents may be a range of 0.1to 40 percent by weight to the whole electro-conductive paste. Under orabove the range, a printing function is lowered.

(6) Dispersion Agents

Dispersion agents such as stearic acid, palmitic acid, myristic acid,oleic acid, lauric acid and the like may be contained in theelectro-conductive paste. If the dispersion agent is conventional, it isnot limited to organic acid. Preferably, a compounding quantity of thedispersion agent may be a range of 0.05 to 10 percent by weight to thewhole electro-conductive paste. Under 0.05 percent by weight, adispersing function becomes poor. Above 10 percent by weight, aresistivity of a light-absorbing side electrode obtained by sintering isincreased.

(7) Other Additives

Additives such as stabilizing agent, antioxidant, ultraviolet absorbent,silane coupling agent, defoaming agent, viscosity adjusting agent andthe like may be contained in the electro-conductive paste withouthindering the function of the present invention.

(8) Method for Producing Solar Cell Device

A method for producing a solar cell device in accordance with presentinvention will be described below in detail.

FIG. 1 is a schematic section view showing an embodiment of a solar celldevice of the present invention. FIG. 2 is a view showing of an exampleof an electrode of the present invention. FIG. 2( a) is a plan viewshowing a light-absorbing side (surface side) of a solar cell device,and FIG. 2( b) is a plan view showing the other side (back side) of thelight-absorbing side of the solar cell device.

A silicon board 1 consists of single crystal silicon or poly crystalsilicon and the like. The silicon board 1 contains oneelectro-conductive type semiconductor impurity such as boron and itsspecific resistance is in the range of about 1.0 to 2.00 cm. Singlecrystal silicon is produced by a pulling up method and poly crystalsilicon is produced by a casting method. The silicon board 1 is obtainedby slicing the silicon ingot produced by the pulling up method or thecasting method to a thickness of 200 μm or less, preferably 150 μm orless. The following is described about p-type silicon board but n-typesilicon board may be used.

The surface of the silicon board 1 is minutely etched by NaOH, KOH,hydrofluoric acid, fluoro nitric acid and the like in order to clean theslicing surface.

After that, an uneven surface (rough surface) which can reduce areflectivity of light may be preferably formed on the surface(light-absorbing side) of the silicon board being a plane of incidenceof light by dry-etching or wet-etching.

Next, a n-type diffusion layer 2 is formed. Phosphorus may be preferablyused as n-type doping element and n⁺ type whose sheet resistance may bein the range of about 40 to 100Ω/□ is obtained. P-n join portion isformed between the p-type silicon board 1 and the n-type diffusion layer2.

The n-type diffusion layer 2 is formed on the light-absorbing side ofthe silicon board by the coating thermal diffusion in which paste-likeP₂O₅ is coated and thermal-diffused, the vapor phase thermal diffusionhaving gaseous POCl₃ as diffusion source, and the ion implantation inwhich P⁺ ion is directly diffused and the like. The depth of the n-typediffusion layer 2 may be in the range of about 0.3 to 0.5 μm.

If a diffusion area has been formed on the unexpected portion, thediffusion area may be removed by etching. As described below, if BSFarea of the back side (the other side of the light-absorbing side) isformed by aluminum paste, since the aluminum paste being p-type dopingagent can be diffused to a sufficient depth with a sufficient density,the influence of a shallow n-type diffusion layer can be ignored and itis not necessary to remove the n-type diffusion layer formed on the backside.

The method for forming the n-type diffusion layer is not limited to theabove methods. For example, by applying a thin layer technology,amorphous silicon membrane hydride, crystalline silicon membraneincluding micro crystal silicon membrane and the like may be formed.Further, i-type silicon board (not shown) may be formed between thep-type silicon board 1 and the n-type diffusion layer 2.

Next, a antireflection layer 3 is formed. SiNx membrane (composition xhas a range such that the middle compound is Si₃N₄), TiO₂ membrane, SiO₂membrane, MgO membrane, ITO membrane, SnO₂ membrane, ZnO membrane andthe like may be used as the material of the antireflection layer 3. Thethickness of the antireflection layer may be adequately selected to thesemiconductor material in order to reproduce no reflection condition toan appropriate light of incidence. For example, a refractive index maybe in the range of about 1.8 to 2.3 and the thickness of theantireflection layer 3 may be in a range of about 500 to 1000 angstroms.

The chemical vapor deposition, the vapor deposition, the sputtering andthe like may be used as the methods for producing the antireflectionlayer 3.

Next, BSF (back surface field) layer 4 may be preferably formed. The BSFlayer is a region in which one electro-conductive type semiconductorimpurity is diffused in high density on the back side of the siliconboard 1. The BSF layer acts a role of preventing a decrease oftransformation efficiency by re-combination of carrier. Boron andaluminum may be used as impurity elements. If the density of impurityelement is made high to become p⁺ type, an ohmic contact can be obtainedbetween the BSF layer 4 and a back electrode 6 as described below.

The BSF layer 4 may be formed at a temperature in a range of about 800to 1000° C. by the thermal diffusion having BBr₃ as a diffusion source.If the thermal diffusion is applied, it is desirable to form a diffusionbarrier such as oxidation layer on then-type diffusion layer 2 inadvance. If the other method using aluminum is applied, after aluminumpaste containing aluminum powder or organic vehicle has been coated,aluminum can be diffused toward the silicon board 1 by sintering at atemperature in a range of about 600 to 850° C. By this method, thediffusion layer can be formed on the coating surface by request and itis not necessary to remove unnecessary diffusion layer on the back side.Furthermore, sintered aluminum may be used as the current collectingelectrode of the back side.

Next, a surface electrode 5 consisting of a bus bar electrode 5 a and afinger electrode 5 b as shown in FIG. 2( a), and a back electrode 6consisting of a bus bar electrode 6 a and a current collecting electrode6 b as shown in FIG. 2( b), are formed on the surface side and the backside of the silicon board 1 respectively.

The surface electrode 5 is formed by a process such that theelectro-conductive paste for forming an electrode of a solar cell deviceof the present invention is coated on the silicon board 1 by a knowncoating procedure, and the paste is sintered for dozens of seconds todozens of minutes at a peak temperature in a range of about 750 to 950°C.

The back electrode 6 may be such that as shown in FIG. 2( b), the busbar electrode 6 a obtained by coating and sintering silver-aluminumpaste containing silver powders, aluminum powders, organic binder,solvent and glass frit, and the current collecting electrode 6 bobtained by coating and sintering aluminum paste containing aluminumpowders, organic binder, solvent and glass frit almost on the wholesurface of silicon board 1 may overlap in part.

After the electro-conductive pastes for forming the surface electrodeand the back electrode have been coated and dried, if both pastes havebeen sintered at the same time, preferably production processes can bedecreased. The order of coating of the above pastes is not limited.

The pattern of electrode by the electro-conductive paste may be aconventional pattern, and a comb type can be adopted in order to collecteffectively current from the solar cell device, as shown in FIG. 2( a),in case of the surface electrode.

(9) Method for Producing the Solar Cell Module

One example of producing the solar cell module using the solar celldevice produced as described above will be described below.

As shown in FIG. 3, the surface electrode and the back electrode of asolar cell device 12 is connected by a wiring 11. The solar cell device12 is sandwiched between a surface filling material 13 consisting oftransparent thermal plastic resin and a back filling material 14consisting of transparent thermal plastic resin. A transparent portion15 consisting of glass is placed on the surface filling material 1 and aback protecting material 16, in which a sheet of polyethyleneterephthalate having good mechanical properties is coated with a film ofpolyvinyl fluoride having good weather resistance, is placed under theback filling material 14. These layered materials may be preferablyheated at an appropriate vacuum furnace to remove air and integrated inone body by heating and pressing. If many solar cell device 12 areconnected in series, the ends of electrodes of the first device and thelast device may be preferably connected to a terminal box 17 beingoutput taking out portion by an output taking out wiring 18.Furthermore, since the solar cell module is left in the open air for along time, it is desirable to cover the circumference of the solar cellmodule with a frame material consisting of aluminum and the like.

EXAMPLES

Examples of the present invention will be described below. The presentinvention is not limited to the following examples and it can beappreciated by a person skilled in the art that an appropriatealteration or revision can be conducted without deviating from theextent of the present invention.

(1) Preparation of Semiconductor Wafer

N-type diffusion layer was formed on the surface of poly crystal p-typesilicon board in which a thickness is 200 μm, an external shape is asquare of 20 mm×20 mm, and a resistivity is 1.5 Ωcm. An antireflectionlayer of SiNx was formed on the n-type diffusion layer. Thus, asemiconductor wafer was prepared.

(2) Preparation of Electro-Conductive Paste

a. Electro-Conductive Paste for Forming BSF Layer and Back CurrentCollecting Electrode

Aluminum powders of 70 parts by weight with average particle diameter ofabout 3 μm, ethyl cellulose (organic binder) of 1 parts by weight,2,2,4-trimethyl-1, 3-pentanediol monoisobutylate (solvent) of 28 partsby weight, and Bi₂O₃—B₂O₃—ZnO containing glass frit of 1 part by weightwith a softening point of about 405° C. were mixed using 3 rolls mill toobtain a paste. As a result, obtained were the electro-conductive pastefor forming the BSF layer and the back current collecting electrode.

b. Electro-Conductive Paste for Forming Back Bus Bar Electrode

Silver powders of 80 Parts by weight with average particle diameter ofabout 1 μm, aluminum powders of 2. 4 parts by weight with averageparticle diameter of about 3 μm, ethyl cellulose (organic binder) of 1parts by weight, 2,2,4-trimethyl-1, 3-pentanediol monoisobutylate(solvent) of 15 parts by weight, Bi₂O₃—B₂O₃—ZnO containing glass frit of1.5 part by weight with a softening point of about 405° C., and stearicacid of 0.1 part by weight were mixed using 3 rolls mill to obtain apaste. As a result, obtained was the electro-conductive paste forforming the back current collecting electrode.

c. Electro-Conductive Paste for Forming Surface Bus Bar Electrode andSurface Finger Electrode

Examples 1 to 17 Electro-Conductive Paste containing Bi-containing GlassFrit

As examples 1 to 7, the alkaline earth metal soap and Te were added toelectro-conductive particles (Ag powders with average particle diameterof 0.4 μm) 86 parts by weight, Bi-containing glass frit with a softeningpoint of about 430° C. (B₂O₃ of 1 to 10 percent by weight, BaO of 1 to10 percent by weight, Bi₂O₃ of 70 to 80 percent by weight, Sb₂O₃ of 1percent by weight or less, and the others (except said matters, ZnO andCaO) of 10 percent by weight or less) of 1 parts by weight, ethylcellulose (organic binder) of 1 parts by weight,2,2,4-trimethyl-1,3-pentanediol monoisobutylate (solvent) of 11 parts byweight, and stearic acid (dispersing agent) of 0.5 part by weight, asshown in table 1. The matter consisting of a group of the aboveelectro-conductive particles, Bi-containing glass frit, ethyl cellulose,2,2,4-trimethyl-1, 3-pentanediol monoisobutylate and stearic acid isreferred to as the first main paste composition hereinafter.

As examples 8 to 11, only magnesium stearate was added to the first mainpaste composition as shown in table 1.

As examples 12 and 13, magnesium stearate and TeO₂ were added to thefirst main paste composition as shown in table 1.

As example 14, only Te was added to the first main paste composition asshown in table 1.

As example 15, only TeO₂ was added to the first main paste compositionas shown in table 1.

As example 16, only Se was added to the first main paste composition asshown in table 1.

As example 17, only ZnTe was added to the first main paste compositionas shown in table 1.

Each composition of examples 1 to 17 was mixed using 3 rolls mill toobtain a paste. The above organic solvent was appropriately added toobtain the paste viscosity of about 300 Pa·s at a time of screenprinting as described below. Thus obtained were the electro-conductivepaste for forming the surface bus bar electrode and the surface fingerelectrode.

Examples 18 to 32 Electro-Conductive Paste containing Ba-containingGlass Frit

As examples 18 to 24, magnesium stearate and TeO₂ were added toelectro-conductive particles (Ag powders with average particle diameterof 0.4 μm) 86 parts by weight, Ba-containing glass frit with a softeningpoint of about 530° C. (B₂O₃ of 20 percent by weight or more, ZnO of 20percent by weight or less, BaO of 40 percent by weight or more, and CaOof 10 percent by weight or more) of 1 parts by weight, ethyl cellulose(organic binder) of 1 parts by weight, 2,2,4-trimethyl-1,3-pentanediolmonoisobutylate (solvent) of 11 parts by weight, and stearic acid(dispersing agent) of 0.5 part by weight, as shown in table 2. Thematter consisting of a group of the above electro-conductive particles,Ba-containing glass frit, ethyl cellulose, 2,2,4-trimethyl-1,3-pentanediol monoisobutylate and stearic acid is referredto as the second main paste composition hereinafter.

As example 25 to 31, only TeO₂ was added to the second main pastecomposition as shown in table 2.

As example 32, only Te was added to the second main paste composition asshown in table 2.

Each composition of examples 18 to 32 was mixed using 3 rolls mill toobtain a paste. The above organic solvent was appropriately added toobtain the paste viscosity of about 300 Pa·s at a time of screenprinting as described below. Thus obtained were the electro-conductivepaste for forming the surface bus bar electrode and the surface fingerelectrode.

<<Contrasts 1 to 6 (Electro-Conductive Paste containing Bi-containingGlass Frit>>

As contrasts 1 to 5, Te of 1 percent by weight and other compounds inplace of the alkaline earth metal soap were added to the first mainpaste composition as shown in table 3.

As contrast 6, no compound was added to the frits main paste compositionas shown in table 3.

Each composition of contrast 1 to 6 was mixed using 3 rolls mill toobtain a paste. The above organic solvent was appropriately added toobtain the paste viscosity of about 300 Pa·s at a time of screenprinting as described below. Thus obtained were the electro-conductivepaste for forming the surface bus bar electrode and the surface fingerelectrode as contrasts.

<<Contrast 7 (Electro-Conductive Paste containing Ba-containing GlassFrit>>

As contrast 7, no compound was added to the second main pastecomposition as shown in table 4.

The composition of contrast 7 was mixed using 3 rolls mill to obtain apaste. The above organic solvent was appropriately added to obtain thepaste viscosity of about 300 Pa·s at a time of screen printing asdescribed below. Thus obtained were the electro-conductive paste forforming the surface bus bar electrode and the surface finger electrodeas contrast.

<<References 1 to 9>>

As references 1 to 3, much magnesium stearate was added to the firstmain paste composition as shown in table 5. As reference 4, muchtellurium was added to the first main paste composition as shown intable 5. As reference 5, much selenium was added to the first main pastecomposition as shown in table 5. As reference 6, much tellurium dioxidewas added to the first main paste composition as shown in table 5. Asreference 7, much zinc telluride was added to the first main pastecomposition as shown in table 5. As references 8 and 9, much telluriumdioxide was added to the second main paste composition respectively asshown in table 5. Each composition of references 1 to 9 was mixed using3 rolls mill to obtain a paste. The above organic solvent wasappropriately added to obtain the paste viscosity of about 300 Pa·s at atime of screen printing as described below. Thus obtained were theelectro-conductive paste for forming the surface bus bar electrode andthe surface finger electrode as references.

TABLE 1 additives parts except Te and TeO₂ by weight kinds parts byweight Te  TeO₂ FF value example 1 Ca stearate 1 1 0 0.665 example 2 Mggluconate 1 1 0 0.667 example 3 Sr stearate 1 1 0 0.569 example 4 Mgstearate 1 1 0 0.692 example 5 Mg stearate 2 1 0 0.735 example 6 Mgstearate 4 1 0 0.645 example 7 Mg stearate 5 1 0 0.520 example 8 Mgstearate 1 0 0 0.459 example 9 Mg stearate 2 0 0 0.498 example 10 Mgstearate 4 0 0 0.480 example 11 Mg stearate 5 0 0 0.462 example 12 Mgstearate 1 0 1 0.730 example 13 Mg stearate 1 0 2 0.750 example 14 Mgstearate 0 1 0 0.669 example 15 — 0 0 2 0.708 example 16 Se 2 0 0 0.642example 17 ZnTe 4 0 0 0.628

TABLE 2 additives parts except Te and TeO₂ by weight kinds parts byweight Te TeO₂ FF value example18 Mg stearate 1 0 0.1 0.611 example 19Ms stearate 1 0 0.5 0.759 example 20 Mg stearate 1 0 1 0.768 example 21Mg stearate 1 0 2 0.771 example 22 Mg stearate 1 0 4 0.780 example 23 Mgstearate 1 0 8 0.690 example 24 Mg stearate 1 0 10 0.611 example 25 — 00 0.1 0.565 example 26 — 0 0 0.5 0.710 example 27 — 0 0 1 0.731 example28 — 0 0 2 0.751 example 29 — 0 0 4 0.756 example 30 — 0 0 8 0.650example 31 — 0 0 10 0.601 example 32 — 0 4 0 0.755

TABLE 3 additives parts except Te and TeO₂ by weight kinds parts byweight Te TeO₂ FF value contrast 1 Mg oxide 1 1 0 0.284 contrast 2 Nistearate 1 1 0 0.430 contrast 3 Cu stearate 1 1 0 0.315 contrast 4 Srcarbonate 1 1 0 0.248 contrast 5 Ca fluoride 1 1 0 0.342 contrast 6 — 00 0 0.270

TABLE 4 additives parts except Te and TeO₂ by weight kinds parts byweight Te TeO₂ FF value contrast 7 — 0 0 0 0.282

TABLE 5 additives parts except Te and TeO₂ by weight kinds parts byweight Te TeO₂ FF value reference 1 Mg stearate 8 1 0 0.276 reference 2Mg stearate 15 1 0 0.370 reference 3 Mg stearate 8 0 0 0.270 reference 4— 0 15 0 short circuit reference 5 Se 15 0 0 short circuit reference 6 —0 0 15 short circuit reference 7 ZnTe 15 0 0 short circuit reference 8 —0 0 15 short circuit reference 9 Mg stearate 1 0 15 short circuit

(3) Printing of Electro-Conductive Paste

Almost whole surface of the back side of the semiconductor waferprepared as described above (1) was coated with the electro-conductivepaste prepared as described above (2)a by the screen printing. Theelectro-conductive paste prepared as described above (2)b was coated onthe said electro-conductive paste by the screen printing in order toform a shape of 6a as shown in FIG. 2( b). Then, after drying at atemperature of 150° C. for five minutes, it was cooled to a roomtemperature by leaving it as it is.

Next, the surface side of the semiconductor wafer prepared as describedabove (1) was coated with the electro-conductive paste prepared asdescribed above (2)c by the screen printing in order to form shapes of5a and 5b as shown in FIG. 2( a). Then, after drying at a temperature of150° C. for five minutes, it was cooled to a room temperature by leavingit as it is.

(4) Sintering

The semiconductor wafer coated with the electro-conductive paste asdescribed above was put in a fast sintering furnace (model PV309) whichhas four heating zones produced by BTU company. The maximum temperatureof the surface of the semiconductor wafer was measured by a temperaturelogger produced by Datapaq company. The said semiconductor wafer wassintered at a temperature of 800° C., which was the maximum temperatureof the surface, for one minute. During this sintering process, aluminumcoated on the back side of the semiconductor wafer diffuses toward thesemiconductor wafer. As a result, BSF layer 4 is formed as shown in FIG.1 and at the same time, the current collecting electrode 6 b is formedas shown in FIG. 1.

(5) Evaluation of Electrical Properties

FF value of the test piece of the solar cell device produced asdescribed above was measured as described below. As a tester, a productof trade name of KST-15Ce-1s produced by kyosin-denki corporation wasused and as a solar simulator, a product of trade name of XES-502Sproduced by kansai-kagakukiki company was used. FF value was obtained bya curve showing a relation between voltage and current. FF values ofexamples, contrasts and references are shown in tables 1 to 5. A largeFF value indicates a high transformation efficiency.

As clearly shown in tables 1 to 5, FF values of examples 1 to 32 of thepresent invention are larger than those of contrasts 1 to 7 andreferences 1 to 3.

By comparison between examples 1 to 7 and examples 8 to 11, FF value ofthe test piece containing appropriate quantity of tellurium andappropriate quantity of alkali earth metal soap is larger than that oftest piece containing no tellurium and only alkali earth metal soap(magnesium stearate).

By comparison between examples 1, 2, 3 and 4, FF value of the test piececontaining magnesium stearate is larger than that of test piececontaining other alkali earth metal soap than magnesium stearate.

By comparison between examples 4 and 12, FF value of the test piececontaining magnesium stearate and tellurium dioxide is larger than thatof test piece containing magnesium stearate and tellurium.

In order to obtain an effect of controlling sintering and increase FFvalue, it is preferably to add magnesium stearate of 1 parts by weightto 5 parts by weight to tellurium of 1 part by weight. As shown in table1, in order to get the maximum FF value in the range of addition ofmagnesium stearate of 1 parts by weight to 5 parts by weigh, it can beunderstood that magnesium stearate of 2 parts by weight and itsapproximate value (1.5 parts by weight to 2.5 parts by weight) ispreferable.

By comparison between examples 12 and 20, examples 13 and 21, examples15 and 28 respectively, FF value of the test piece containingBa-containing glass frit tends to be larger than that of test piececontaining Bi-containing glass frit.

In case of the test piece containing Ba-containing glass frit, bycomparison between examples 25 to 31, FF value higher than 0.75 can beobtained by adding tellurium dioxide of 2 parts by weight to 4 parts byweight. Further, in case of the test piece containing Ba-containingglass frit, by comparison between examples 18 to 24, if both telluriumdioxide and magnesium stearate of 1 parts by weight are contained, FFvalue higher than 0.75 can be obtained at addition of tellurium dioxideof 0.5 parts by weight and FF value can reach the maximum by addingmagnesium stearate of 1 parts by weight and tellurium dioxide of 1 partsby weight to 4 parts by weight. Further, in case of the test piececontaining Ba-containing glass frit, in example 32, FF value higher than0.75 can be obtained at addition of tellurium of 4 parts by weight andno addition of magnesium stearate.

By comparison between examples 4 to 7 and references 1 to 2, whentellurium of 1 part by weight and excessive magnesium stearate areadded, an effect of controlling sintering of silver particles can beobtained but FF value is decreased due to increase of resistance.

By comparison between examples 8 to 11 and reference 3, when excessivemagnesium stearate is added, an effect of controlling sintering ofsilver particles can be obtained but FF value is decreased due toincrease of resistance.

By comparison between contrasts 1 to 5, if the electro-conductive pastecontains tellurium of 1 part by weight and no organic compoundsincluding alkali earth metal, FF value is decreased.

In references 4 to 9 in which tellurium powder, selenium powder,tellurium dioxide and zinc telluride powder are much added, theelectrical conduction effect of the antireflection layer by thefire-through was very large and a short-circuit between the surfaceelectrode and p-type silicon occurred. As a result, it was not possibleto measure FF value.

The summary of evaluation on FF values of examples, contrasts andreferences is as follows:

(1) FF value is increased by adding appropriate quantity of magnesiumstearate, a metal with a low melting point and a compound affiliatedwith a metal with a low melting point to the electro-conductive paste.

(2) FF value of the electro-conductive paste containing appropriatequantity of tellurium and appropriate quantity of alkali earth metalsoap is larger than that of the electro-conductive paste containing notellurium and only alkali earth metal soap. Magnesium stearate ispreferable as alkali earth metal soap.

(3) When magnesium stearate and tellurium are contained in theelectro-conductive paste, if magnesium stearate of 1.5 parts by weightto 2.5 part by weight is added to tellurium of 1 part by weight, it isprobable that FF value of the electro-conductive paste reaches themaximum.

(4) FF value is increased if tellurium dioxide is contained in theelectro-conductive paste, even though no organic compound includingalkali earth metal is contained the electro-conductive paste. In thiscase, FF value becomes larger by using Ba-containing glass frit as glassfrit.

(5) FF value of the electro-conductive paste containing magnesiumstearate and tellurium dioxide is larger than that of theelectro-conductive paste containing magnesium stearate and tellurium. Inthe case, if Ba glass frit is used as glass frit and tellurium dioxideof 1 part by weight to 4 parts by weight is added to magnesium stearateof 1 part by weight, it is probable that FF value reaches the maximum.

(6) FF value of the electro-conductive paste containing other materialthan organic compound including alkali earth metal is low, even thoughtellurium is contained.

As shown in tables 1 and 2, FF values of the test pieces of the solarcell device of examples 1 to 32 are larger than those of the test piecesof the solar cell device of contrasts 1 to 7 and references 1 to 3.Therefore, it can be understood that contact resistance of the testpieces of the solar cell device of examples 1 to 32 are smaller thanthose of the test pieces of the solar cell device of contrasts 1 to 7and references 1 to 3.

INDUSTRIAL APPLICABILITY

The present invention is suitable for electro-conductive paste forforming an electrode of a solar cell device and a solar cell device.

EXPLANATION OF NUMERALS

1 silicon board

2 n-type diffusion layer

3 antireflection layer

4 BSF (back surface field) layer

5 surface electrode

5 a bus bar electrode

5 b finger electrode

6 back electrode

6 a bus bar electrode

6 b current collecting electrode

11 wiring

12 solar cell device

13 surface filling material

14 back filling material

15 transparent portion

16 back protecting material

17 terminal box

18 output taking out wiring

21 semiconductor board

22 diffusion layer

23 antireflection layer

24 back electrode

25 surface electrode

1.-16. (canceled)
 17. An electro-conductive paste for forming anelectrode of a solar cell device, comprising: an electro-conductiveparticle; an organic binder; a solvent; a glass frit; and an alkalineearth metal soap, wherein an average particle diameter of theelectro-conductive particle is in a range of 0.1 to 15 μm, theelectro-conductive particle is present in an amount of 65 to 95 percentby weight, and the alkaline earth metal soap is present in an amount of0.1 to 5 percent by weight.
 18. The electro-conductive paste for formingan electrode of a solar cell device according to claim 17, wherein thealkaline earth metal soap is calcium stearate, magnesium stearate,strontium stearate, or magnesium gluconate.
 19. A solar cell devicecomprising: a diffusion layer formed on a light-absorbing side of asemiconductor board; an antireflection layer and a surface electrodeformed on the diffusion layer; and a back electrode formed on theopposite side of the light-absorbing side of the semiconductor board,wherein the surface electrode is formed by printing and sintering theelectro-conductive paste according to claim 17 on the antireflectionlayer.
 20. A method for producing a solar cell device comprising:forming a diffusion layer on a light-absorbing side of a semiconductorboard; forming an antireflection layer on the diffusion layer; printingthe electro-conductive paste according to claim 17 on the antireflectionlayer; forming a back electrode by printing an electro-conductive pasteon the opposite side of the light-absorbing side of the semiconductorboard; forming a surface electrode by sintering the electro-conductivepaste printed on the antireflection layer to the diffusion layer; andforming the back electrode by sintering the electro-conductive paste onthe opposite side of the light-absorbing side of the semi-conductorboard.